To make this boss easier you can use a little trick like I did in this video, if you stay in the outer area of the room (next to the throne is the best spot) the phantom knight will not move any closer and will not use the attack where he throws his axe. Just run close to him until he decides to use the axe stomp attack (he very rarely uses it though, and it seems to be random) and then attack him in the back (really close or it won't work) after his axe gets stuck in the ground. You need to hit him four times to defeat him (the fight takes a long while, hence why in the video I cut a few minutes off the battle, but it's a lot easier to beat him this way -- otherwise it's near impossible)